both...left...both...right...both...Why?

[WhiskeyPapa]

How many people debating this on this thread have actually found a bad mag?

I've found three. None were subtle when the switch was placed on the bad mag.

The most obvious wasn't a bad mag at all. One of the plug wires had cracked and fallen completely off the plug.

The other two, the mag drop was enough you'd notice it no matter what technique you used.

I've had 400RPM drop on a mag. Was simply fouled. Leaning hard and running up in the low 2000s fixed it (this is in a plane that hadn't flown in a couple months)...

 

[AcroGimp]

In your Yak, do you run up to high RPM just before shutting down? We did this religiously in Russia.

Yes, 70% for 10-20 seconds to make sure the scavenge pump pulls as much oil out of the case as possible, then idle, verify stable ~24% and cage the Mags (and don't forget the Mags in the rear when de-planing).

Mine does not have an aftermarket oil shutoff so I open the manifold drain and so long as I pull it through prior to start or get 6 blades once a week when not flying it is fine.

'Gimp

 

[Stewartb]

How many people debating this on this thread have actually found a bad mag?

I had a significant mag drop that turned out to be a failure of the mechanic to put a wrench on the lower spark plug caps on one side after he'd finger tightened them buttoning up after an annual. They vibrated loose a couple of flights later. Stuff happens.

 

[kgruber]

Thing is that if there was fouling a good way to handle it in the boonies would have been high RPM run up (2000+) with heavy leaning… a couple minutes of that and the mags could have been running clean again… would have led to a safer take off…

Only on a rental airplane. I'd NEVER do that to a plane I owned.

 

[Henning]

Only on a rental airplane. I'd NEVER do that to a plane I owned.

I would do a super lean max RPM step taxi cruise around the lake to clear them though.

 

[kgruber]

I would do a super lean max RPM step taxi cruise around the lake to clear them though.

It must be the super human way I operate an airplane, but I've never had a bad mag check due to leading. Never!

 

[dtuuri]

If you have a typical Bendix key switch that is L-R-Both-(Start)...

Um, nope.

dtuuri

 

[Henning]

Um, nope.

dtuuri

What do you mean 'nope' because it's R-L-Both not L-R,-Both? Irrelevant, the relevant factor is the switch is not Off-L-Both-R as some older switches were. With toggle switches you go through Both between mags as well. The only issue is that the instructions do not particularly make sense to apply to these type of switches. It doesn't make an operational difference regardless which way you go about it, you'll just wear out the switch twice as fast one way than the other is all. If someone wants to waste the money to make themselves feel safer, please, be my guest.

 

[Dan Thomas]

It doesn't make an operational difference regardless which way you go about it, you'll just wear out the switch twice as fast one way than the other is all. If someone wants to waste the money to make themselves feel safer, please, be my guest.

Wearing switches out is not a result of checking as per POH. All the switches I have ever replaced were due to:

(A) starter switch contact burnout (most often in ACS switches that were not in compliance with AD 93-05-06 and had not been inspected and lubricated every 2000 hours and the required diode on the starter contactor wasn't there. That AD applies even to new switches, not just old ones. The only ACS switches exempt are those without a Start position;

or (b) worn-out locks caused by the key going in and out thousands of times, typically on flight school airplanes, so that the lock pins wore and let the key fall out. Worn keys also cause the lock pins to not quite line up when the key's in and they then get worn;

or (C) Bendix switches that failed the test prescribed by AD 76-07-12, another often-overlooked AD. Amost all Cessnas and Pipers have either Bendix or ACS switches in them. If you buy a Cessna switch, it's an ACS switch. I often find both these ADs overlooked, or previous mechanics have declared them N/A by misidentifyng the switch, or have not properly complied with the AD requirements.

The only part in the switch that would see any significant wear from being turned on and off would be the detent, and I have never had one fail. It's the most robust part of the switch. The mag shorting contacts only give trouble when they get really old and the grease hardens and messes with conductivity.

Dan

 

[dtuuri]

Um, nope.

dtuuri

What do you mean 'nope' because it's R-L-Both not L-R,-Both?

Yep. I have next to no confidence that those opining here realize how the switch is actually labeled, which makes it impossible to tell what they really do. "Operationally", I want to avoid a backfire not save the switch from wearing out. Some engines start on the left mag which is the nearest position to Both. Some pilots forget to set it to both when the start button is released. Then when they do their runup, if they are in the habit of going two "clicks", they'll kill the engine and reflexively turn the key back on and probably blow the muffler. If they are in the habit of going just one click first they have a better chance of finding their mistake because the engine will run rough due to the fouled plugs from taxiing on one mag. IMO, a properly taught mag check includes what to do if the engine dies while switching, i.e., don't turn the key back, bring the throttle to idle, wait for the rpm to come down to 1000 before turning the key back on (or let it die and restart). I've done that without causing a backfire. YMMV.

dtuuri

 

[Henning]

Yep. I have next to no confidence that those opining here realize how the switch is actually labeled, which makes it impossible to tell what they really do. "Operationally", I want to avoid a backfire not save the switch from wearing out. Some engines start on the left mag which is the nearest position to Both. Some pilots forget to set it to both when the start button is released. Then when they do their runup, if they are in the habit of going two "clicks", they'll kill the engine and reflexively turn the key back on and probably blow the muffler. If they are in the habit of going just one click first they have a better chance of finding their mistake because the engine will run rough due to the fouled plugs from taxiing on one mag. IMO, a properly taught mag check includes what to do if the engine dies while switching, i.e., don't turn the key back, bring the throttle to idle, wait for the rpm to come down to 1000 before turning the key back on (or let it die and restart). I've done that without causing a backfire. YMMV.

dtuuri

How do you possibly prevent a back fire by adding more switching motions onto the exact same switching motions?

BTW, you don't have to let it die to prevent a fuel pop in the exhaust (it's not a 'back fire'), just pull the mix to ICO and then turn the key on, followed by putting the mixture in.

 

[dtuuri]

How do you possibly prevent a back fire by adding more switching motions onto the exact same switching motions?

I don't understand you. All I can say is, if you were ever a CFI you'd understand me.

dtuuri

 

[Henning]

I don't understand you. All I can say is, if you were ever a CFI you'd understand me.

dtuuri

The only discrepancy involved is in switching the switch back to both between L&R on a switch where both doesn't exist between them. The anomaly in the check lists stems from the fact that 2/3rds of the switching systems you have to go through both to go from L-R.

In other words, all you are doing is adding a proceedure that is completely unnecessary and increases your chances of making an error than decreasing it. You go from Both, L, R, L, Both to Both, L, Both, L, R, L, Both.

Now as a CFI, would you care to explain the mechanics involved that improve the test with the extra switching moves?

 

[dtuuri]

BTW, you don't have to let it die to prevent a fuel pop in the exhaust (it's not a 'back fire'), just pull the mix to ICO and then turn the key on, followed by putting the mixture in.

That'll work too. If I'd have called it an "afterfire" it would have confused people unnecessarily.

dtuuri

 

[MBDiagMan]

If you have a typical Bendix key switch that is L-R-Both-(Start) the issue of not going back to both between them is an issue of wear and tear on the switch. They're known junk to begin with, there's is no reason to unnecessarily wear them out. There is no reason not to just go R-L then back to both. There is absolutely no extra information to be gained by going back to both in between L & R.

Doing something stupid because it is written there doesn't make you cautious, it makes you stupid.

Come on Henning! Say what you REALLY feel.

 

[MBDiagMan]

In 11 years I've never found ANY issues while preflighting and/or running up my 182 except for an occasional loose screw here and there.

I guess I'll just start skipping all the rest of it. That'd save me at least 30 minutes per trip.

Kick the tires and light the fire baby!





How does killing the engine by turning the mags off tell a person anything more than a mag check does? If you do a mag check and have a significant RPM drop on each mag, then both P leads are good. Conversely, killing the engine with the mags doesn't tell you anything about the health of the individual mags so I'd argue that NO you're not better served by doing it...you're actually better served (and get far more information) by doing a standard mag check.

Note to self. Don't fly with this guy!

 

[dtuuri]

The only discrepancy involved is in switching the switch back to both between L&R on a switch where both doesn't exist between them. The anomaly in the check lists stems from the fact that 2/3rds of the switching systems you have to go through both to go from L-R.

In other words, all you are doing is adding a proceedure that is completely unnecessary and increases your chances of making an error than decreasing it. You go from Both, L, R, L, Both to Both, L, Both, L, R, L, Both.

Now as a CFI, would you care to explain the mechanics involved that improve the test with the extra switching moves?

Don't you have an anchor to check or something? I think you've drifted too far out to sea.

dtuuri

 

[Henning]

Don't you have an anchor to check or something? I think you've drifted too far out to sea.

dtuuri

So, you don't have an answer, thank you, that is because one does not exist. It's just stupidity being taught due to just reprinting a single procedure to cover three different switching systems, one of which the exact details arent necessary.

 

[dtuuri]

So, you don't have an answer, thank you, that is because one does not exist. It's just stupidity being taught due to just reprinting a single procedure to cover three different switching systems, one of which the exact details arent necessary.

References, please. It makes sense to have clean plugs before checking each ignition system. Which one you do first depends on your experience or lack of it. FWIW, I'm not one to blindly follow manufacturer's recommendations when it comes to operations. They don't know my mission requirements. In the past, I made an emergency checklist for the Citation that greatly improved on Cessna's. After using it in the sim at FSI and Simuflite, the word must have got back to Cessna because they rewrote the factory checklist in the same way I did. Imitation is the sincerest form of flattery.

dtuuri

 

[NoHeat]

If you use an engine analyzer that logs data, it is desirable to go back to BOTH, between the RIGHT and LEFT. That's so that after the flight, you will be able to clearly identify, in a graph of the EGT data, when the mag was in the RIGHT, when it was in LEFT, and when it was in BOTH.

Same for in-flight mag checks.

 

[Henning]

References, please. It makes sense to have clean plugs before checking each ignition system. Which one you do first depends on your experience or lack of it. FWIW, I'm not one to blindly follow manufacturer's recommendations when it comes to operations. They don't know my mission requirements. In the past, I made an emergency checklist for the Citation that greatly improved on Cessna's. After using it in the sim at FSI and Simuflite, the word must have got back to Cessna because they rewrote the factory checklist in the same way I did. Imitation is the sincerest form of flattery.

dtuuri

Wow, if you need to go back to both to clean plugs between mags, you just failed runnup.

 

[jesse]

BTW, you don't have to let it die to prevent a fuel pop in the exhaust (it's not a 'back fire'), just pull the mix to ICO and then turn the key on, followed by putting the mixture in.

No student will be fast enough to do that, nor will they remember the steps.

Whenever I teach a student how to check mags I always have them intentionally switch it to off and tell them to let it shutdown then restart it. I usually have them do this two or three times throughout their training to burn into their mind to just let it shutdown.

There is very little time on many engines from the point you go to off to the point that the engine quits spinning. I don't think even non-students are going to be fast enough to do what you said unless they were expecting to make the mistake.

 

[Henning]

No student will be fast enough to do that, nor will they remember the steps.

Whenever I teach a student how to check mags I always have them intentionally switch it to off and tell them to let it shutdown then restart it. I usually have them do this two or three times throughout their training to burn into their mind to just let it shutdown.

There is very little time on many engines from the point you go to off to the point that the engine quits spinning. I don't think even non-students are going to be fast enough to do what you said unless they were expecting to make the mistake.

Understood. The point wasn't to teach them to use that method, the point was that two differnt methods can produce identical operational results by understanding the nature of the process. By the logic of your teaching, teach the simplest method that produces the desired result, would that also support teaching the B-L-B-L-R-L-B method, or B-L-R-L-B? Which introduces less chances of accidently turning to OFF?

 

[dtuuri]

Wow, if you need to go back to both to clean plugs between mags, you just failed runnup.

If you have clean plugs to begin with running them in a cool, sooty, rich cylinder makes 'em dirty. Why wouldn't you want to keep 'em clean? To "save" the switch? To "save" time?

dtuuri

 

[Henning]

If you have clean plugs to begin with running them in a cool, sooty, rich cylinder makes 'em dirty. Why wouldn't you want to keep 'em clean? To "save" the switch? To "save" time?

dtuuri

You don't seem to understand the conditions in the combustion chamber. If you have a cool sooty exhaust fouling the plus on a single mag check, you will have failed the RPM drop on the mag you were just using.

 

[dtuuri]

You don't seem to understand the conditions in the combustion chamber. If you have a cool sooty exhaust fouling the plus on a single mag check, you will have failed the RPM drop on the mag you were just using.

Maybe your A&P license is more current than mine? Care to explain?

dtuuri

 

[RoscoeT]

If you have clean plugs to begin with running them in a cool, sooty, rich cylinder makes 'em dirty. Why wouldn't you want to keep 'em clean? To "save" the switch? To "save" time?

dtuuri

I will ask this again -

This plug clearing explanation makes no sense. When you move from both to the right mag, you are grounding the left mag off. If you then go straight to left, you are turning the left mag back on and grounding the right mag off. The time spent running on one mag is no different than if you went back to both after checking the right mag. That's what can cause fouling - time spent running on one plug. Someone please explain how going straight from right to left increases the time spent running on a single plug.

 

[Henning]

Maybe your A&P license is more current than mine? Care to explain?

dtuuri

You should still be getting a complete burn on a single mag if it is functioning properly. Using only a single flame front changes the timing, and the peak ICP, of the combustion event which is why you lose RPM if there is sufficient load, but there should be absolutely no sooty byproduct. If the conditions in the combustion chamber are such that there is a sooty by product, your RPM drop will be more than the allowable drop, hence it will have failed the test before you have a chance to turn the switch back to both the first time.

 

[dtuuri]

I will ask this again -

I have no idea what you're talking about. Sorry.

dtuuri

 

[dtuuri]

You should still be getting a complete burn on a single mag if it is functioning properly.

If that were true there would be no drop on one mag and Lycoming wouldn't say go back to Both to minimize the fouling.

Using only a single flame front changes the timing, and the peak ICP, of the combustion event which is why you lose RPM if there is sufficient load, but there should be absolutely no sooty byproduct.

I need more than your opinion and Lycoming's (for one) agreement. Refs, please.

If the conditions in the combustion chamber are such that there is a sooty by product, your RPM drop will be more than the allowable drop, hence it will have failed the test before you have a chance to turn the switch back to both the first time.

Again, show me the refs.

dtuuri

 

[Henning]

If that were true there would be no drop on one mag and Lycoming wouldn't say go back to Both to minimize the fouling.

I need more than your opinion and Lycoming's (for one) agreement. Refs, please.



Again, show me the refs.

dtuuri

Hmmm, probably the best amassed public access data for combustion processes is from the GAMI/TAT folks in Oklahoma. The same data is elsewhere as well, they didn't revolutionize combustion physics, but they did refine it in the aircraft engine application and have done a good job of presenting the information to the pilot community. Please, ask them if it makes a difference to go to Both between mags, outside of data logging purposes. I promise, the engine could really not care less, and has no affect on test values.

 

[dtuuri]

Hmmm, probably the best amassed public access data for combustion processes is from the GAMI/TAT folks in Oklahoma. The same data is elsewhere as well, they didn't revolutionize combustion physics, but they did refine it in the aircraft engine application and have done a good job of presenting the information to the pilot community. Please, ask them if it makes a difference to go to Both between mags, outside of data logging purposes. I promise, the engine could really not care less, and has no affect on test values.

You ask 'em. I'm happy with doing it the same way it's always been done. Ask 'em to include Lycoming's approval for it while they're at it.

dtuuri

 

[RoscoeT]

I have no idea what you're talking about. Sorry.

dtuuri

Likewise

 

[dtuuri]

Sorry, I guess I made it difficult by including my original post right there in the response.

Yeah, the one that makes no sense.

dtuuri

 

[RoscoeT]

Yeah, the one that makes no sense.

dtuuri

You mean the one asking the highly complex question related to how going back to both before checking the other mag decreases the time (NOT) spent running on one mag??

 

[dtuuri]

You mean the one asking the highly complex question related to how going back to both before checking the other mag decreases the time (NOT) spent running on one mag??

It's a pointless question that has nothing to do with anything.

dtuuri

 

[RoscoeT]

It's a pointless question that has nothing to do with anything.

dtuuri

OK then I'd like to hear your explanation for why going back to both decreases plug fouling. You brought it up.

 

[Henning]

You ask 'em. I'm happy with doing it the same way it's always been done. Ask 'em to include Lycoming's approval for it while they're at it.

dtuuri

Fine, you are welcome to pray at the alter if you please, you won't cause harm, so please, be my guest.

 

[dtuuri]

OK then I'd like to hear your explanation for why going back to both decreases plug fouling. You brought it up.

Higher temps, complete combustion, less deposits on the 'cold' electrodes. The EGT goes up because the the mixture is still burning after it gets in the exhaust manifold. The CHT goes down because there's not as much fire in the head anymore. Unburned fuel wetting the plugs richens the chambers and causes dirtier plugs on the next stroke. I thought it's common knowledge. But I'm ready to learn something new, just not ready to go looking for it myself.

dtuuri

 

[Henning]

Higher temps, complete combustion, less deposits on the 'cold' electrodes. The EGT goes up because the the mixture is still burning after it gets in the exhaust manifold. The CHT goes down because there's not as much fire in the head anymore. Unburned fuel wetting the plugs richens the chamber and causes dirtier plugs on the next stroke. I thought it's common knowledge. But I'm ready to learn something new, just not ready to go looking for it myself.

dtuuri

Exactly, it's still burning, but it still burns completely as long as the mixture was in the nose of the LEL-UEL range. The plugs don't know the difference.